Wikipedia:Reference desk/Archives/Science/2013 June 21

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June 21

How does a paper airplane fly in the space station?

For a aircraft such as a paper airplane to fly in the air above the ground, the lift force upward should balance the weight force downward. The gravity is almost zero in the space station. How does a paper airplane fly in the air of zero-gravity environment? My guess is that without gravity, there will be frictional force downward due to the air, but I'm not sure how the paper plane behave. I have found an article titled "Zero-gravity flight of paper airplanes" by John Bain, NRC/NASA in 2000, but I can't access it. Could someone tell me what this article talked about? Thanks! Armeria wiki (talk) 03:52, 21 June 2013 (UTC)

If it's in free fall, why would the frictional force be downward rather than just backward? It seems to me that the airplane would go straight ahead, the direction affected only by the shape of the plane and the odd air current. Clarityfiend (talk) 04:36, 21 June 2013 (UTC)

My guess is that since there is no downward gravity force on the paper plane, the paper plane with certain initial velocity in the air will get an upward lift force, and it will fly forward and upward. There will be a backward air frictional force to gradually reduce the forward velocity of the paper plane, and a downward air frictional force since the paper plane will move upward. Armeria wiki (talk) 05:03, 21 June 2013 (UTC)

How about video demonstration: [1] -- Scray (talk) 05:06, 21 June 2013 (UTC)

Thanks! Could you please tell me what the video demonstrated? My internet connection is not good. Thanks! Armeria wiki (talk) 05:48, 21 June 2013 (UTC)

The paper, Zero Gravity Flight of Paper Airplanes (AIAA, 2000). Briefly, the paper describes a series of educational outreach programs for Houston-area students of various academic levels between 1998 and 2000; they design an experiment to launch and measure trajectories of a small toy airplane inside the NASA "Vomit Comet" KC-135A aircraft, in a ground laboratory, and in a computer simulation.

From the data collected, I'd say the paper airplane (actually, a foam space-shuttle toy and a student-made mechanical launcher apparatus) produces very little lift; its behavior is almost perfectly ballistic, with slight air resistance. So, in 1-G, the airplane travels in a just-barely asymmetric parabola; and in the 0-G of the KC-135A, the airplane flies in almost a perfect straight line. The paper delves into some more advanced math, and calculates the lift and drag more accurately; and studies the nonzero pitching moment of the paper airplane. And, there is some extra math to deal with the complexity of the rotating coordinate frame (because the KC-135A is not a perfect "0-gravity" environment - it is neither stationary, nor perfectly fixed acceleration, nor in a constant orientation); so I should say, the trajectories of the model aircraft inside the airborne lab need to be interpreted carefully.

It is worth stating that lift is a very nontrivial quantity to calculate: just because something has wings does not guarantee that it generates lift at any particular attitude. Nimur (talk) 05:34, 21 June 2013 (UTC)

After reading a little farther: During a final flight in year 2000, an improved launcher was used, capable of launching paper airplanes faster than before - at 25 ft/sec (17 mph) in ground and airborne experiments. In this experiment, airborne (0-gravity) trajectories actually show significant upward drift indicating that the model aircraft is pitching up and generating lift. Nimur (talk) 05:52, 21 June 2013 (UTC)

Thanks! I found this paper by John Bain by can't access it. Could their foam space-shuttle toy fly as well as a normal paper airplane with normal gravity? Armeria wiki (talk) 05:54, 21 June 2013 (UTC)

The specific model was a WhiteWings Space Shuttle foam airplane. I have never played with that specific space-shuttle toy, but I've spent enough time with a variety of foam and other model airplanes to know how they fly - they're quite a bit heavier and more rugged than an ordinary paper-airplane. One of the biggest problems facing a paper-airplane is its lack of inertia and its very light wing loading due to its very low mass - so a paper airplane is incredibly susceptible to gusts, turbulence, drag due to cross-sectional area, and because it is so easy to turn over, a paper airplane is easily upset to unusual attitudes that are unsuitable for stable flight. The foam airplane toys are much heavier, and while that weight does work against them, it also has advantages over paper in terms of stability and speed.

Because the WhiteWings Space Shuttle is roughly shaped like ... the Space Shuttle... it's not a great aerodynamic platform. Its wings are short and stubby, and its total wing area is small. This is probably a contributing factor to my earlier comment - its flight trajectory looks pretty close to ballistic in ground-tests - in other words, not much different from the flight-path of a brick. This is actually how NASA pilots described the Space Shuttle Orbiter: "un-flyable" and brick-like. Nimur (talk) 06:21, 21 June 2013 (UTC)

It seems that the paper airplane in air with zero gravity will get a upward lift force and fly up and forward. Since in the zero gravity space, there is no real "up" or "down" direction. What "up" means for the paper airplane is the direction perpendicular to the surface of its wings. So will the paper plane fly in a big circle trajectory if its initial velocity is large enough and the room of the space station is large enough once the lift force perpendicular to the instantaneous velocity of the paper airplane serves as a centripetal force? Armeria wiki (talk) 06:15, 21 June 2013 (UTC)

That is one very plausible trajectory - if a few conditions are met. The aircraft must have positive lift, and a positive pitching moment. The air - atmosphere - in the space-station's laboratory can't get too turbulent - which is a big if. The rate of pitching and the lift must be stable. Other effects - like rolling, yawing, and lateral movement - and drag - need to be negligible. If these conditions are met, the aircraft can maintain steady-state flight with a circular trajectory. Nimur (talk) 06:25, 21 June 2013 (UTC)

The other answers here are probably better, but just intuitively from my very low-grade efforts: my impression is that a crude paper airplane has lift because it inclines a bit upward. And the reason why it inclines upward is that it is heavy in the rear end. So with no weight, there's no lift... Wnt (talk) 08:00, 21 June 2013 (UTC)

Eh...well, not really. Lift (for a more or less planar surface like a paper airplane wing) is a function of angle of attack. Certainly, any paper plane that's trying to gain altitude from level flight needs a positive angle of attack. But in a zero g environment, it's not entirely clear which way is "UP"...so how does the airplane "know" what its' angle of attack is? When you think about it - it's when the velocity vector isn't parallel to the plane of the wing...then the wing deflects the air so that it is parallel - and that results in lift. So if you throw your paper plane such that it has a not-parallel-to-velocity attitude at launch - then even in zero g, it'll develop a flight path that deflects away from the original launch vector...which is the nearest thing to "lift" that you can reasonably talk about. Pretty soon, the pressure of deflecting that air downwards will cause a rotation towards making the wing parallel to the new velocity vector - at which point, the plane flies in a straight line. I suspect that this is the primary effect with paper planes in one-g environments too. You don't see paper planes flying along in a nose-up attitude to any great degree.

Remember, a "zero g" environment is just "free-fall" - and the paper airplane is in free fall even if you launch it on the surface of the earth. SteveBaker (talk) 14:19, 21 June 2013 (UTC)

The last sentence on "free-fall" of paper airplane on the surface of the earth sounds not right. In the space, the space station including the air and the paper airplane inside is in free fall as a whole, and thus the paper airplane feels zero gravity. This is not the case on the earth. Armeria wiki (talk) 06:08, 22 June 2013 (UTC)

Again, you are mixing up two parameters! Airspeed is totally independent of weight. In free-fall on Earth, you often expect an airspeed to be about equal to your downward velocity, but that is incorrect and an unsafe assumption. A counter-example of this is a updraft or downdraft - an aircraft measures an airspeed relative to the sea of air it is moving through; it may perceive no vertical motion relative to the air; but if the entire column of air is moving up or down, the aircraft may gain or lose altitude without changing any other fllight parameters. When you consider a weird environment like a flight-test inside a space-station in microgravity, you have to be very careful not to mix up your parameters. Motion relative to the airmass determines angle of attack, and therefore helps determine lift, but it has absolutely nothing to do with weight. Nimur (talk) 16:29, 22 June 2013 (UTC)

To Nimur, I think I didn't mix up those two parameters.Armeria wiki (talk) 02:29, 23 June 2013 (UTC)

Then you should have no problem! An airfoil in free-fall inside a space-station is no different than an airfoil in free-fall on Earth. If you conclude that the airfoil on Earth has a lot more wind rushing past it, then it's got an effective airspeed! Nimur (talk) 19:19, 24 June 2013 (UTC)

I don't agree with you on the free-fall. I think a paper airplane in free-fall inside a space-station is different from one in free-all on Earth. Consider an extrem case when the initial speed of the paper airplane is zero, it will stay at the initial position when it in free-fall in a space-station but it will freely fall down to the surface of Earth.Armeria wiki (talk) 08:17, 25 June 2013 (UTC)

(ec with Steve)

Wnt is describing flow deflection, which does produce lift (and drag). But, that type of lift depends on angle of attack and the shape of the object - not the object's weight. The angle of attack for a piece of paper is detemined by pitch, and pitch is determined by the pitching moment, which can be affected by weight - it is usually calculated by summing the weights and their moment arms. But it's not true that the absence of weight automatically yields zero pitching moment; nor that it yields zero lift... these are distinct parameters. For example, a trim tab on a paper airplane would still work in microgravity.

While Wnt's observation probably is valid for (paper) airplanes in normal conditions, it would start to break down in weird cases like microgravity. Nimur (talk) 14:30, 21 June 2013 (UTC)

I'm reminded of a competition that was run on my companies' summer picnic for the pre-teen kids of employees. They were given a single sheet of paper and told that they had 5 minutes to make a paper airplane that would fly further than the competition. My son (then 10 years old) scrunched his paper into a tightly wadded ball (he called it "Death Star") and easily won the first round of the contest ahead of about 50 other kids. There was considerable unrest amongst parents - and the contest organizers were thrown for a loop. They hastily changed the rules for the second round to prohibit scrunched up balls - so Oliver folded his paper into the thinnest possible wingless needle-like shape and easily won the second round too. The conclusion being that drag is overwhelming more important than lift for paper planes! SteveBaker (talk) 14:10, 21 June 2013 (UTC)

clostridium

If a wound is contaminated by clostridium species, it may favour developing of complications,,,one example I think is Gas Gangrene,, anyone knows other examples ??????? — Preceding unsigned comment added by 201.220.215.11 (talk) 04:09, 21 June 2013 (UTC)

Is this a request for medical advice, or a general question about bacterial infections and their outcomes? If the former, it is not permitted under Wikipedia Reference Desk guidelines and must be closed without response. If the latter, then referenced responses are welcome. Have you read Clostridium and Gas gangrene? Edison (talk) 04:15, 21 June 2013 (UTC)

Tetanus is another possible consequence -- have you looked at our Clostridium article? Looie496 (talk) 06:32, 21 June 2013 (UTC)

Melancholia

Is it plausible for an asteroid to follow a trajectory like that in the movie (enter the Solar System at high speed on the opposite side from the Earth; closely approach the Sun while remaining eclipsed thereby from the Earth at all times; fly by the sun at high speed, presumably with a close perihelion, while still remaining invisible to the naked eye; cross the orbits of Mercury and Venus and eclipse Antares while still remaining invisible to the naked eye; make a close fly-by of the Earth on the night side (close enough to illuminate it to civil twilight level); and finally, slingshot completely around the Earth and hit it from behind)? My intuition tells me this is probably an impossible trajectory, but could someone verify? Also, I remember a few years back there was a scare about a real asteroid (albeit one that entered the Solar System from the same side that Earth was on) which was predicted to slingshot around the Moon and hit the Earth (just like in the movie), but missed -- could someone please refresh my memory regarding the asteroid's catalog number, and whether it inspired the movie? 24.23.196.85 (talk) 05:48, 21 June 2013 (UTC)

Not a good answer this time, but: My first impression is that if a rogue planet comes heading for the Earth, odds are it will hit somehow, because odds are there's a rocket engine poking out the back of it. :) But for this exercise I suppose we should put aside active holographic camouflage and rapid acceleration.

For this exercise we should assume infinite bad luck. God (the god of bad sci-fi, to be precise) is clearly out to get these people. To begin with, that means that the rogue planet may execute a successful aerobraking maneuver through the outer layer of the Sun that leaves it with just enough energy to reach Earth's vicinity, despite having fallen into the solar system from great distance.

The premise is apparently that it is a stealth planet, practically invisible even on the same side of the sun, so provided we can swallow that (basically, a zero albedo) it is hard to picture seeing it on the far side, provided it doesn't occlude somebody's favorite star (odds?). (Presumably the dark layer is stripped away by passing through the Sun's outer layer and/or by passing within Earth's Roche limit, if it does)

Planet or no, it should obey some of the basic rules of orbital mechanics - ignoring the initial velocity being wrong, if it's at the orbit of the Moon it should take a month to orbit (or else move away, not nearer); if it's 24,000 miles away it should be in geosynchronous orbit, and if it is grazing the tops of the clouds it might indeed be in hours. I'm not sure if the figure in the article is in hours or days, but you could make some arguments based on that. Wnt (talk) 08:16, 21 June 2013 (UTC)

The trajectory as described in the article is possible, if you take liberties in interpreting the description: in order for a "large rogue planet" to fly by Earth, turn around, and hit, one of the extremes of the orbit (ether perihelion or aphelion) would need to be just outside Earth's orbit. The problem with the film as described is one of plausibility: anyone with a backyard telescope and a basic knowledge of orbital mechanics would be able to predict the collision months or years in advance, so there's no way it wouldn't be known that the end of the world was at hand.

Incidentally, the occultation of Antares is a likely point of impossibility: the article doesn't say how long it lasts, but stellar occultations don't usually last more than a few minutes. --Carnildo (talk) 01:29, 22 June 2013 (UTC)

Well, if it entered the Solar System from the outside, it can't have an aphelion anywhere near the Earth's orbit, can it? And its perihelion was well inside Mercury's orbit as per the dialogue: "The scientists said it wouldn't hit Mercury and it didn't, and they said it wouldn't hit Venus and it didn't". So this would be a second point of impossibility. (Which is precisely what I suspected -- a planet that enters the Solar System from the outside and then passes close to the Sun would be going much too fast when it reaches Earth to slingshot around it and hit.) 24.23.196.85 (talk) 01:59, 22 June 2013 (UTC)

This is why Wnt measured aerobraking. If you are exclusively following gravity-based orbital mechanics, then yes, the planet will necessarily have a hyperbolic orbit. Someguy1221 (talk) 02:07, 22 June 2013 (UTC)

Well, yes, but -- in the movie, the rogue planet has a bright blue color, which suggests that its surface is mostly water -- and, if it went through the Sun's upper layers, all the water would evaporate, wouldn't it? 24.23.196.85 (talk) 03:16, 22 June 2013 (UTC)

Maybe it was a solid ball of ice when it was going through outer space... :) (I haven't seen the movie, though) I still feel very suspicious of the path going around the Earth, away, then back again, but with the Moon it's a three-body problem, so it's hard to rule out divine malice. Wnt (talk) 04:43, 22 June 2013 (UTC)

The rogue planet is much bigger than the Moon (in fact, it's shown as roughly Earth-sized), so the Moon can't influence its path significantly -- if anything, the Moon itself would be pulled out of orbit. 24.23.196.85 (talk) 20:01, 22 June 2013 (UTC)

I should also mention the possibility of an imperfect-knowledge scenario - perhaps the planet was guided by aliens, or the government misrepresented the facts to reduce panic just in case by some miracle the collision didn't happen, etc. I'd have to have seen the movie to come up with one with confidence. My personal feeling is that imperfect-knowledge scenarios greatly enhance the experience of watching very good science fiction (for example, understanding that Neo woke up from the machine world at the end of The Matrix Revolutions) and do something to salvage bad sci fi (i.e. here, I think), but is of little use on sci-fi of intermediate quality in which a limited number of ideas are presented in a straightforward way. Wnt (talk) 17:40, 22 June 2013 (UTC)

And my personal feeling is that since the main theme of the movie is the psychology of depression, the director used the asteroid as a symbol thereof and picked the facts accordingly (e.g. the planet "hiding behind the Sun"; its orbit, which would first pass the Earth by and give cause for optimism, and then come back and crush all hopes; its very name, "Melancholia"; and even its bright blue color (as in, "feeling blue" -- get it?)). So they really tweaked the facts in order to get the most symbolic mileage out of it -- just like that part in Deer Hunter where Nick played Russian roulette for months on end, even though in real life a person's chances of surviving it for more than two weeks are essentially nil. 24.23.196.85 (talk) 20:01, 22 June 2013 (UTC)

Speakers and headphone burn in

Is there truth in the fact that speakers and headphones sound better after a 100 hours or so of use? Why is this? Clover345 (talk) 08:15, 21 June 2013 (UTC)

According to this [2], yes - some headphones sound very slightly different after they've been used for some number of hours. "Better" is subjective...so is the number of hours and so is which brands of headphones sound better. I presume speakers are the same. There are mechanical components (the moving cone and the magnets it's connected to) that may well have less friction after they've had a few hours of wear - or the flexible cone gets more flexible after it's been bent back and forth a few million times. It's plausible that there could be some change like that.

HOWEVER: Audiophiles are notorious for spouting bullshit about what they can and can't hear. We've all seen those complete and utter nonsense discussions of $500 gold plated USB cables and how people claim that there is more "warmth" or "presence" through a $500 cable than a $5 one. To anyone who understands the first thing about digital signal processing, this is an incredibly stupid claim. Either the numbers in the audio data make it through the cable or they don't...there is no possibility of them losing high frequencies or something! You can transfer a picture from your camera to your laptop through a USB cable - and then do the same thing by removing the Micro-SD card from camera and plugging it into the computer - then compare the two binary files for that picture and they are byte-for-byte identical every single time. Same deal with the audio. It's complete nonsense that switching out a perfectly functional cable for a gold-plated one could have any effect at all unless the old cable was so bad that it didn't work at all.

Check out (for example) the "Monster Digital" USB cable - no longer priced at $500 as it once was - but still a somewhat exhorbitant $25. Here is a typical customer review: "Gives me a little more head room to work with recording vocals I notice.". The guys at Guitar Center say "Advanced design and construction maintain proper impedance to reduce jitter, resulting in better focus, clarity, and extended dynamic range." - complete and utter bullcrap!!

The whole field of audio is a very sad affair. Back 30 or 40 years ago, equipment didn't reproduce sounds very well and there was immense scope for tireless geeks to fiddle around with different components to try to get it better, to write long magazine articles about it - to argue at the pub about whether the such-and-such speakers are "warmer" than the so-and-so kind with or without the blarfatron-2000 pre-amp. They all had an immense amount of fun and became world-class experts. But with the arrival of digital audio, signal-processing and sophisticated computer design for things like speakers - you get essentially perfect quality from even the very cheapest devices - there is really nothing left to geek-out about. So we have all of these aging audiophiles, just desperate to find something to make them sound more expert than some kid who just listens to MP3's on the stock headphones of his $20 MP3 player. So you hear huge amounts of utter hogwash like "getting better headroom on vocals" with an expensive gold plated digital cable. If we ever get cars that can all do 0-60 in three seconds, drive 200mph and get 400mpg in utter comfort and which only cost $1000, the exact same thing will happen with car nuts...and that's about the scale of what digital did to most aspects of audio.

So while there is some evidence that there is some tiny change in the frequency spectrum of older headphones, I'm 100% skeptical about whether the human ear can tell - or whether it matters or not - or whether they get better or worse with age.

SteveBaker (talk) 13:45, 21 June 2013 (UTC)

Your points on digital audio cables are apt, and audiophiles do like to go on... that being said, mp3 is highly lossy, even compared to a CD. CD's also have compression that analog signals do not. Whether you can hear it or not, there is a clear scientific/engineering basis for why some people might say a good audio tape sounds better than an iTunes mp3, played through the same system. I personally only notice on songs with lots of range in pitch, loudness, etc. but my ears aren't that great ;) SemanticMantis (talk) 14:52, 21 June 2013 (UTC)

Just to devils-advocate a little, the cable linked speaks of charging faster. Is it possible that faster charging of a speaker will change its performance? Also, I may TOTALLY misunderstand what the role of the USB cable is here, but if you're transmitting digital data to the speaker plus power and the speaker deciphers it at the far end, then won't the speaker be drawing power in a pattern that directly matches the waveform of the music, i.e. be sending a pulsed analog signal back up to the computer power supply, where all manner of impedance effects on it etc. can then have some faint influence on the originating speaker? Wnt (talk) 15:54, 21 June 2013 (UTC)

The "pulsed waveform" you describe would be variations in the power and ground wires of the USB cable. The waveform has a frequency of 20 kilohertz or below, and the wires are reasonably directly connected to the computer's power supply. The digital signals traveling down the cable are on the two data wires, have a frequency in the high hundreds of megahertz, bear no resemblance to the final analog output, and have incredibly complicated circuitry between them and any possible connection to the power supply, most of which acts to filter out anything as low-frequency as audio. Yes, there will be some influence, but the influence will be so small that even the best-equipped lab couldn't measure it.

The "charges portable electronics faster" claim just means they didn't skimp on the power-supply wires. A lot of cheaper USB cables do to reduce costs, and the resulting resistive losses mean the cables can't actually deliver as much current to whatever's plugged in as they're supposed to. --Carnildo (talk) 01:47, 22 June 2013 (UTC)

The final audio output of the speaker is actually the physical motion of a magnet in a field to push on the room air - it shouldn't be at hundreds of megahertz, though if the speaker is decoding digital sound from a USB cable it must contain some electronics that do draw power at that rate also. (I think ... I've never actually used a speaker remotely like what's being described here) Wnt (talk) 04:39, 22 June 2013 (UTC)

Although some audiophile stuff is nonsense, there is definitely a lot of variation in headphone fidelity. I have Sennheiser HD 280 Pro headphones ($100) and Philips SBC HP250 headphones ($10 used) and some musical details that I can hear on the Sennheiser headphones are just not audible on the Philips, even after I know they're there. I've also listened to expensive professional open headphones and it was a lot more like being at a live concert; the sound seemed to be out in the room rather than near my ears. Cheaper and closed headphones introduce a lot of distortion, and it's certainly plausible that the details could change over the life of the headphones. One would hope that that would be less true of the more expensive ones, but I don't know.

Streaming digital audio is not quite trivial because you have to worry about desynchronization of the sending and receiving clocks. This page appears to show a substantial degradation of the output signal in one USB mode versus another because of clock jitter (the green charts at the bottom). I'm pretty sure an expensive USB cable is not going to help with that, though. -- BenRG 20:21, 22 June 2013 (UTC)

crystals and molecular bonds?

From our article Crystal.

[...] new atoms can very easily attach to the parts of the surface with rough atomic-scale structure and many dangling bonds.

I am not sure I understand what are these "bonds" which attach the atoms. Are they similar to molecular bonds? If so, can we consider a crystal to be a single, very large, molecule? --Lgriot (talk) 11:12, 21 June 2013 (UTC)

A variety of bonds are possible which explains why crystals can have so many properties. In a salt crystal the bonds are ionic bonds which explains why salts can so easily be dissolved in water. In a diamond the bonds are covalent bonds which explains why diamonds are so strong. In water ice the bonds are hydrogen bonds which are weaker than either ionic or covalent bonds explaining the low melting point of water. In hydrocarbons the bonds are Van der walls bonds explaining the even lower melting points. Dauto (talk) 13:50, 21 June 2013 (UTC)

Thanks.--Lgriot (talk) 14:28, 21 June 2013 (UTC)

Resolved

Exercise

So I'm a little overweight, and I want to exercise but I find running not just physically, but also mentally frustrating, and it doesn't make me feel fresh or anything, unless I run for an hour or more. I also get bored when I run and listening to music doesn't help much either. So I want to do something that both takes a lot of energy and is fun, and I want to be able to do it on my own. Do you have any ideas? I like going hiking but I guess it doesn't take much energy, and some people say it's bad for your knees, is that true?--Irrational number (talk) 22:16, 21 June 2013 (UTC)

See "5BX" and "XBX".—Wavelength (talk) 22:31, 21 June 2013 (UTC)

If you have access to a pool (or lake, etc.), then swimming is a good option. One advantage is you avoid the impacts from running, which can cause injuries. I also like that I don't get sweaty and stink after swimming (unlike with running). You're also less likely to overheat while swimming, unless the water is hot. StuRat (talk) 22:40, 21 June 2013 (UTC)

Here are the exercises that I find useful: (1) running; (2) swimming; (3) bicycling, especially up hills; (4) hiking on hills, especially if you carry a heavy pack. I personally like bicycling best most of the time, because you get to see the most per unit of effort. A lot depends on what sort of environment you live in, though. Looie496 (talk) 22:57, 21 June 2013 (UTC)

Exercise intensity also matters. Reading what you write, I suspect that you run rather slowly. Unless you are old (say over 60) you should try to run faster (check with your doctor to make sure this is safe to do for you) so that running is a heavy exercise. If you can run for more than about 40 minutes at the right training pace, you should be extremely fit. You should be well above the power/weight ratio of 3 Watt/kg, so if you weigh 80 kg then 240 Watt for half an hour should not be a problem, it should be close to your regular exercise intensity. Count Iblis (talk) 23:27, 21 June 2013 (UTC)

Depending on your circumstances bicycle commuting is good exercise while simultaneously getting you somewhere you needed to go anyway (and possibly helping to save the planet by reducing motor vehicle emissions. Mitch Ames (talk) 12:35, 22 June 2013 (UTC)

I get worried when people automatically connect exercise with losing weight. There are very good reasons to exercise for general health reasons - but for actually burning calories, it's highly overrated. For people with a scientific mind and a desire to lose weight, I strongly recommend The Hacker's Diet - which has a companion work about taking sensible exercise. What I like about it is that it starts from solid scientific premises and exposes the math for you to understand. http://www.myfitnesspal.com/exercise/lookup is a great way to understand how many calories you'll burn for various exercises...it reveals that I can burn 500 calories by running 5 miles in an hour - but cutting out two snacks saves just as much. Running makes me so hungry that I eat more and actually put on weight as a result! SteveBaker (talk) 17:34, 22 June 2013 (UTC)

Well, this is very individual, though. Some people eat for reasons other than hunger, and exercise often helps resolve these.

My experience is that you can achieve serious and long-term weight loss with exercise alone — but it takes a lot of it. Think on the order of an hour a day, at fairly high intensity. When I was a grad student/postdoc, I was able to fit that into my schedule. In industry it's harder. --Trovatore (talk) 17:40, 22 June 2013 (UTC)

I agree, I also think that the whole idea of counting calories is misleading, as it ignores the fact that the body will regulate the amount of calories it burns via feedback mechanisms that make sure you don't eventually starve to death if you eat just a little less every day. Conversely, this means that if you eat a little more every day, your weight shouldn't increase on the long term. What exercise does is to let this feedback mechanism work better, so that your weight becomes more independent of the amount of calories you eat. The more of a couch potato you are, the more susceptible to weight gain you'll become. Count Iblis (talk) 19:24, 22 June 2013 (UTC)

Also, Steve, my experience is that there is a little upward spike when you start an exercise program, could be up to five pounds. I attribute it to water retained by inflamed muscles, though I don't have any proof of that. To get a good clean signal, you at least need to average over a couple of months. --Trovatore (talk) 19:53, 22 June 2013 (UTC)

I suspect that when people talk about doing exercise to "lose weight", what they really want to do is lose fat. Moderate exercise can burn off fat but increase muscle (slightly, I'm not talking bodybuilding). Since muscle is denser than fat, it is possible that exercise can increase weight, but the resultant body looks better/slimmer, and is healthier. Mitch Ames (talk) 02:11, 23 June 2013 (UTC)

• Rowing machines and elliptical trainers are much better with low impact on the legs, and you can watch TV, or listen to fast-paced music like Stayin' Alive. μηδείς (talk) 18:53, 22 June 2013 (UTC)
  • Squash. --StewieCartman (talk) 10:42, 25 June 2013 (UTC)

I was going to suggest some form of Martial Arts "takes a lot of energy and is fun,.." but "... I want to be able to do it on my own" seems to nix that. But believe me karate or similar, if you dedicate yourself to it is interesting, fun and takes heaps of energy. Karate kata are exercises that can be performed individually, but you need to be taught them first. Not sure if I can recommend Tae Bo.

If worried about your knees then I suggest just fast walking, with good shoes and if possible on grass not concrete. If you have been sedentary I strongly suggest consulting you doctor before engaging in any unusual exercise. 220 of ^Borg 16:03, 25 June 2013 (UTC)